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KYRA PHILLIPS, CNN ANCHOR: All this week, we've been focusing on infidelity in America. And some researchers who study human behavior suggest that something primitive within men and women pushes them to cheat. So Kathy Slobogin left humans behind and she went to the animal king done for some answers.

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JANE TOLLINI, ZOOKEEPER: We're going to be taking a train ride through the zoo. Penguins are probably the most romantic animal in the zoo.

SLOBOGIN: But looks are deceiving, according to zookeeper Jane Tollini.

TOLLINI: There's a penguin we named Joan Collins who would literally sashay in front of the burrows, where pairs are sitting on their eggs. And the males, you would watch them come out. And they would go out, schtup Joan in the hall. And then pick up a palm frond and carry it back to the nest, acting like, that's where I'd been.

SLOBOGIN: And penguins aren't the only ones. It turns out, most animals are cheating on the side.

DAVID P. BARASH, CO-AUTHOR, "THE MONOGAMY MYTH": I'd have to say a species that doesn't cheat is exceedingly rare.

SLOBOGIN: Zoologist David Barash and his wife, psychiatrist Judith Lipton, have studied monogamy in the animal kingdom. You may be surprised by how little of it they found.

BARASH: I know of one species of animal that I can be fairly -- -- in fact, quite confident -- is monogamous. And that is a flatworm that lives as a parasite in the intestines of fish.

SLOBOGIN: In fact, the desire to stray in both animals and humans may be deeply imprinted on our psyches, part of the instinct to survive.

Anthropologist Helen Fisher. HELEN FISHER, ANTHROPOLOGIST: And what Darwin said was, if you have four children and I have no children, you live on and I die out. So who breeds, who reproduces, who passes their genes to the next generation survives. Men seem to have a tendency to sleep around with a lot of different women, so that they could pass more of their genes into the next generation.

SLOBOGIN: And women?

FISHER: When a woman sleeps around, she can collect extra resources for the children that she has. So, through millions of years of having genetic payoffs to both men and women, we evolved whatever it is in the male and female brain to be somewhat adulterous.

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SLOBOGIN: Monogamy in the animal kingdom is so rare that Barash and Lipton, the authors of "The Monogamy Myth," say that those romantic Hallmark cards with pictures of swans or other types of lovebirds should more adequately feature the flatworm. Swans may mate for life, but they're not necessarily faithful to their mate -- Kyra.

PHILLIPS: All right, Kathy, so how did scientists find out about all the fooling around in the animal kingdom?

SLOBOGIN: Well, the clue was DNA. They looked at the offspring of birds and found that the mother bird's offspring weren't necessarily the father bird's offspring. About 40 percent of the babies were sired by someone other than the mother's mate.

And it is not just the male birds that are straying. Researchers actually put little radio transmitters on the mother birds and found out they were sneaking off in the morning to a neighboring nest for a tryst and then slipping back home. It really shocked the scientists.

PHILLIPS: Oh, my goodness. All right, so, if cheating is so common in the animal kingdom and it is also has a genetic function, is monogamy unnatural?

SLOBOGIN: Well, what's natural is the impulse to look around. And it doesn't mean you don't love your spouse.

As one expert said to me, if you're breathing, you're going to be attracted to other people. But Barash and Lipton make the point that we differ in a significant way from other animals. We have the capacity to say no. We have the capacity to consider the consequences. So we have the capacity for monogamy. That doesn't mean it's easy.

Tomorrow, we look at a surprising phenomenon, the number of happy couples who are hit by infidelity -- Kyra.

PHILLIPS: All right, Kathy Slobogin, thank you.

The complete package, "CNN PRESENTS: Infidelity," that's on Sunday night at 8:00 Eastern right here on CNN. TO ORDER A VIDEO OF THIS TRANSCRIPT, PLEASE CALL 800-CNN-NEWS OR USE OUR SECURE ONLINE ORDER FORM LOCATED AT www.fdch.com